SS-peptides (Szeto-Schiller Peptides) are peptides that have antioxidant activity in mitochondria and are being developed for use in pharmaceutical drugs (patent documents 1 and 2).
The method whereby a peptide is synthesized by conventional solid-phase synthesis from a commercial amino acid derivative and cut from the resin, and then the untreated peptide is purified by preparative liquid-phase chromatography (patent document 1) is known as a method for producing one of the SS-peptides, SS-31 (D-Arg-Dmt-Lys-Phe-NH2). Nevertheless, the solid-phase synthesis of a peptide and purification by preparative liquid-phase chromatography are not suited to an industrial scale and therefore, mass synthesis of SS-31 by the above-mentioned production method is difficult.
On the other hand, the following peptides having intramolecular Arg(Z)2 are known as intermediates of peptide synthesis:
(1) Dipeptide: 2-(R)-(2-(4-(1-n-propyl)phenyl)ethyl)-1,5-pentanedioic acid 1-(1-arginine, N-methylamide)amide is obtained as a colorless solid by silica gel column chromatography purification (patent document 4).
(2) Dipeptide: (R)—N,N-bis(Cbz)-N-diphenylacetyl-(R)—N-[1-(4-hydroxyphenyl)-ethyl]arginine amide is obtained as a white powder by silica gel column chromatography (methylene chloride/ethyl acetate) purification (patent document 5).
(3) Tripeptide: Boc-D-Arg(Z)2-Phe-MeβAla-OPac is obtained as a white solid by silica gel column chromatography (chloroform/methanol=100/1) purification followed by crystallization from hexane (patent document 6).
(4) Tripeptide: Boc-D-2-NaI-Arg(Z)2-2-NaI—NH2 is obtained as a solid by silica gel column chromatography (chloroform/methanol=50/1) purification followed by crystallization from ethyl acetate (non-patent document 1).
(5) Tetrapeptide: Boc-Arg(Z)2-Gly-Asp(Bn)-Ser(Bn)-OBn is obtained as a white powder by silica gel column chromatography (chloroform/methanol=99/1) purification (patent document 3).
(6) Tetrapeptide: [4-tert-butyl-2 (R)-[3-(4-methoxyphenyl)propyl]succinyl]-L-tryptophan-L-alanine-L-N,N-bis(benzyloxycarbonyl)arginine-N-methylamide is obtained as a milky white solid by silica gel column chromatography (chloroform/methanol=200/1) purification (patent document 7).
(7) Pentapeptide: MeO-D-Trp-Leu-Val-Arg(Z)2-Val-NHBoc is obtained by silica gel column chromatography (ethyl acetate/hexane) purification (non-patent document 2).
As described above, a peptide having intramolecular Arg(Z)2 is generally purified by silica gel column chromatography. There are almost no examples of successful crystallization. Taking into consideration the purity required of a peptide product, silica gel column chromatography is indispensable at the present time.